Hello and welcome.
You do not NEED to be the owner of the content, but you'll be most rewarded if you are.
If you are not, you can always be introducing an artist for example, giving proper credits, some artists won't allow sharing of their work even if you give credit. And remember that you are earning on someone's elses art/work and that will bring copyright laws and image rights to the table.
"Viral videos" probably won't get you much on Steemit.
For news for example, for sharing links and article summaries (without original content) there is the tag #newslink
Enjoy Steemit !